Peel potatoes and cut them into 1" pieces. Put in a saucepan, cover with cold salted water and bring to a boil. Cook until tender. Drain and return the pan to the heat for a few minutes to dry the potatoes. Rub potatoes through a course sieve into a large bowl. Season with salt & pepper before mixing in the nutmeg and the flour. Beat the egg and stir it into the potatoes, usaing a wooden spoon; blend until smooth.
Turn the mixture onto a floured working surface and knead lightly. With lightly floured hands, shape the mixture into a roll about 1" thick. Cut into 24 even pieces and shape them into balls.
Bring a large pan of water to a boil. Drop in the gnocchi, a few at a time, and simmer for 5 minutes. When cooked, the gnocchi will rise to the surface. Lift them out carefully with a slotted spoon and put in a warm buttered serving dish.
While the last of the gnocchi are cooking, melt the 4 tbsp of butter and pour over gnocchi. Sprinkle with the grated cheese before serving.
Or: Can also be served with tomato sauce instead of butter.